New Mexico Geological Society Annual Spring Meeting — Abstracts

The San Juan Basin contains one of the few records of superposed Late Cretaceous and early Paleocene terrestrial faunas and floras. However, the ages and durations of the Upper Cretaceous Naashoibito Member of the Kirtland Formation and the lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation are poorly constrained, thus limiting the ability to correlate their fossil records to the global time scale and hindering efforts to examine such factors as diversity and survivorship of various groups of plants and animals across the K-Pg boundary. The ages of the Naashoibito and the Ojo Alamo are especially contentious and age interpretations range from Campanian to early Paleocene.

Here we present new geochronologic results that combine magnetostratigraphy and ⁴⁰Ar/³⁹Ar dating of detrital sanidine from sedimentary units and sanidine phenocrysts from a volcanic ash to constrain the ages of the Naashoibito Member, the Ojo Alamo Sandstone, and the lower Nacimiento Formation. Coupled detrital sanidine dates, magnetostratigraphy, and mammal biochronology indicate that the Naashoibito correlates to chrons C30n – C29r, suggesting a relatively short depositional history These results indicate that the youngest Cretaceous sedimentary rocks in the San Juan Basin were likely deposited within the last ~400 kyr of the Cretaceous and that there is a significant unconformity in the Kirtland Formation between the Naashoibito Member and the underlying De-Na-Zin Member. Magnetostratigraphy of the Ojo Alamo Sandstone and the Nacimiento Formation at both De-na-zin Wash and near Mesa de Cuba indicate that there is no evidence for diachroneity at the base of the Nacimiento across the basin. Magnetostratigraphy and a ⁴⁰Ar/³⁹Ar sanidine date from an ash within the Nacimiento demonstrate that the middle Puercan Land Mammal Age (Pu2) interval zone began within ~400 kyr of the K-Pg boundary. A probable volcanic ash coincident with the first occurrence of Pu3 mammals tentatively suggests that Pu2 was only ~150 kyr long. These dates and our magnetostratigraphy indicate that the Ojo Alamo was deposited in chron C29r and the lower Nacimiento in chrons C29r – C28r.This new geochronology helps to constrain the ages of the first occurrence of the Pu2 and Pu3 faunas in the San Juan Basin and indicates that deposition of basal Paleocene strata in the basin began <300 kyr after the K-Pg boundary.